What is the legal test for a defendant to appeal against a finding that he has been found not guilty on appeal?

We appointed counsel to represent defendant on appeal. Counsel filed an opening brief setting forth the facts of the case and requesting that this court review the record to determine whether there are any arguable issues on appeal. (People v. Wende (1979) 25 Cal.3d 436.) Defendant was advised of his right to file a supplemental brief within 30 days of the date of filing of the opening brief. More than 30 days elapsed, and we received no communication from defendant.